Sunday, February 28, 2010

Inside the Jovians

How do you gather information about the interior of planets that lack a solid surface and that are so different from the earth? You combine the best observational data you have with testable, constrained speculation known as theoretical modeling. Doing just this, astronomers have concluded that the interiors of all four jovians consist largely of the elements found in their atmospheres: hydrogen and helium. As we go deeper into the planet, the gases, at increasing pressure and temperature, become liquid. In the case of Jupiter, it is believed that the hot liquid hydrogen is transformed from molecular hydrogen to metallic hydrogen and behaves much like a molten metal, in which electrons are not bound to a single nucleus, but move freely, conducting electrical charge. As we shall see in just a moment, this state of hydrogen is likely related to the creation of Jupiter’s magnetosphere—the result of its powerful magnetic field. Astronomers are less confident about the nature of the very core of Jupiter, though most believe that it is a rocky core the diameter of the earth. Of course, the incredible temperatures and pressures at this depth in Jupiter mean that the material in the core might behave very differently from materials that we have studied on Earth. Saturn’s internal composition is doubtless similar to Jupiter’s, though its layer of metallic hydrogen is probably proportionately thinner, while its core is slightly larger. Temperature and pressure at the Saturnine core are certainly less extreme than on Jupiter.Uranus and Neptune are believed to have rocky cores of similar size to those of Jupiterand Saturn surrounded by a slushy layer consisting of water clouds and, perhaps, theammonia that is largely absent from the outer atmosphere of these planets. BecauseUranus and Neptune have significant magnetospheres, some scientists speculate that the ammonia might create an electrically conducting layer, needed to generate the detected magnetic field.Above the slushy layer is molecular hydrogen. Without the enormous internal pressures present in Jupiter and Saturn, the hydrogen does not assume a metallic form.